EP0419104B1 - Method and apparatus for detecting level of molten metal - Google Patents

Description

• This invention relates to a method of and an apparatus for detecting the position of the level of molten metal, and particularly to, but not limited to, a method of and an apparatus for detecting the position of the level of molten metal within a mold in operation of a continuous casting apparatus.
• There are known the following methods of detecting the position of the level of molten metal:
1) Use of a float
• A float is floated on a molten metal surface and the position of this float is detected by a bar, a chain or the like.
2) Use of optical (optoelectric conversion) technique
• Since the brightness changes at a boundary between the molten metal surface and the container such as a mold, this boundary is measured by triangulation technique using for example, a sensor array, television cameras or the like.
3) Use of ultrasonic wave
• The distance to a surface of molten metal is measured by the time required for an ultrasonic wave irradiated on the surface of the molten metal to return to an original position after reflection by the surface of the molten metal.
4) Use of radioactive ray
• A radioactive ray is transmitted through the molten metal in a diagonal direction, and the surface level of the molten metal is detected from the amount of attenuation of the radioactive ray during transmission through the molten metal.
5) Use of immersed electrode
• The level of the molten metal is detected by turning on or off of an electric circuit provided by the molten metal and an immersed electrode.
6) Use of thermocouple
• Several thermocouples are buried in the outer surface of the wall of the container of the molten metal, and the level of the molten metal is indirectly detected from the changing point in the temperature distribution measured by the thermocouples.
7) Use of electromagnetic induction
• For example, in the Japanese Patent Publication No. 55-16749, a coil which is long in the depth direction of the container is provided on the outer surface of the wall of the mold and connected to one side of an impedance bridge circuit. Since the temperature of the mold wall changes with the change of the level of the molten metal within the mold, resulting in change of the specific resistance of the mold wall, the level of the molten metal is detected from the change of the eddy current produced in the mold wall due to the change of the specific resistance of the mold wall.
• The conventional methods for detecting the level of the molten metal, however, involve the following problems:
1) Use of a float
• The float is subjected to corrosion by the high-temperature molten metal, and also the slug, molten metal or the like sometimes adheres to the float, thereby changing the specific gravity of the float, resulting in the necessity of recalibration.
2) Use of optical (optoelectric conversion) technique
• When smoke, dust or the like is present or when the slug (which generally provides a low brightness and looks black) is floated on the surface of the molten metal, the measurement is difficult. Further when the optical sensor portion is soiled by this smoke or the like, or when the molten metal is at a high temperature and the light is refracted by the heat waves, the measurement may involve errors.
3) Use of ultrasonic wave
• When the molten metal is at a high temperature, the air is waved (changed in its density) by the heat, causing complicated refraction of sound to disable the measurement.
4) Use of radioactive ray
• There is a problem with safety, and the radiation source and the detector must be placed in a special space.
5) Use of immersed electrode
• The electrode is consumed greatly in the high-temperature molten metal and thus cannot be used for a long time.
6) Use of thermocouple
• Since the container for the high-temperature molten metal is made of firebricks, its heat conduction is poor. Thus, not only is the measurement delayed but also the detection precision is poor. Moreover, it is difficult to bury the thermocouples in the container wall, or to exchange a broken thermocouple with a new one.
7) Use of electromagnetic induction (Japanese Patent Publication No. 55-16749)
• The temperature change at the molten metal surface is indirectly measured as the temperature change on the mold wall. However, the temperature change on the mold wall at the molten metal surface is less sharp, thus causing measurement errors.
• Moreover, since the mold is cooled, it is more difficult to detect this temperature change as an impedance change, thus inevitably causing measurement errors.
• Further, in a recently developed continuous casting apparatus using a belt drive type mold, as disclosed in, for example, Japanese Patent laid-open No. JP-A-60-152347, it is required to continuously detect the level of the molten metal within the mold during operation. However, any of the conventional methods as above-mentioned is difficult to be used for the purpose, because the surface of the molten metal exposed to the top of this type of mold is small and various auxiliary devices for operation of the mold are disposed on the outside of the mold, resulting in insufficient physical space.
• US-A-4 138 888 discloses an arrangement for electro-magnetically measuring the level of and/or distance to molten metal contained in a container. A transmitter coil and a receiver coil are arranged in a cigar-shaped torpedo ladle wagon for molten metal. The transmitter coil is located in the uppermost part of the torpedo ladle wagon and is square and consists of one single horizontal lead turn. The receiver coil is located in the side part of the torpedo ladle wagon and is rectangular, the receiver coil also including one single planar lead turn.
• An AC power supply power supply feeds the transmitter coil via a current sensing circuit with current of constant strength. The receiver coil is connected to the primary winding of a transformer of which the secondary winding is connected to the input of an amplifier. The output of the amplifier is connected to one input of a differentiating circuit of which the other input is fed from the AC power supply via a circuit arranged to provide adjustable control of amplitude and/or phase so that in the absence of liquid material in the container the basic signal from the differentiating circuit is substantially zero. The output of the differentiating circuit is connected to the input of a high or band pass filter circuit. The output of the filter circuit is connected to signal processing means including first and second amplitude sensing circuits.
• The first amplitude sensing circuit is arranged to produce a first control signal when the amplitude of the signal from the filter circuit has exceeded a first predetermined level and thereafter dropped markedly below a second predetermined level. The other amplitude sensing circuit is arranged to produce a second control signal proportional to the top amplitude value of the signal from the filter circuit.
• It is an object of the invention to provide a method of and an apparatus for detecting the level of a high-temperature molten metal in a mold from outside of the mold without directly contacting the molten metal, without being much affected by the atmospheric conditions such as smoke, dust or heat and without the necessity of relatively large space.
• It is another object of the invention to provide a method of and an apparatus for continuously detecting the level of the molten metal in a mold of a continuous casting apparatus during operation thereof.
• The present invention provides a method of detecting the level of molten metal existing within a mold, comprising the steps of:
     disposing the coils of at least one set of coils comprising a transmission coil and a receiving coil so as to oppose to each other with the mold interposed therebetween;
     applying an AC voltage to said transmission coil of the said at least one set to produce alternating magnetic flux so that at least a part of said alternating magnetic flux passes through said mold and the molten metal, if any, and reaches said receiving coil of the said at least one set; and
     determining the level of said molten metal by sensing both the voltage value and the phase of the AC signal induced in said receiving coil of the said at least one set by the part of the alternating magnetic flux.
• The present invention also provides apparatus for detecting the level of molten metal
• (1) existing within a mold comprising:
     at least one set of coils comprising a transmission coil and a receiving coil disposed so as to oppose each other with said mold interposed therebetween;
     voltage applying means for applying AC voltage to said transmission coil of the said at least one set to produce alternating magnetic flux so that at least a part of said alternating magnetic flux passes through the mold and the molten metal, if any, and reaches said receiving coil of the said at least one set; and
     means for determining the level of the molten metal by sensing both the voltage value and the phase of an AC signal induced in said receiving coil of the said at least one set by said alternating magnetic flux.
• A way of carrying out the invention will now be described by way of example with reference to the accompanying drawings, in which:
• Fig. 1 is a block diagram of a level detection apparatus not embodying the invention.
• Fig. 2 is a block diagram of a modification of the apparatus of Figure 1.
• Fig. 3 is a graph showing the relation between the voltage induced in the receiving coil and the level of the molten metal in the arrangement of Fig. 1.
• Fig. 4 is a graph showing the relation between the voltage induced in the receiving coil and the level of the molten metal in the arrangement of Fig. 2.
• Fig. 5 is a block diagram of an embodiment of the invention.
• Fig. 6 is a graph showing the relation between the output of the phase detector and the level of the molten metal.
• Fig. 7 is a graph showing the comparison between the output of the low-pass filter and a value of the output multiplied by the cosine of the phase difference in the arrangement of Fig. 5.
• An apparatus for the detection of the level of the molten metal within a mold will be described with reference to Fig. 1. Referring to Fig. 1, there are shown side walls 2, 3 of a mold made of thin steel plate in, for example, a continuous casting apparatus, and molten steel 1. In the continuous casting apparatus, as is well known, the molten steel is poured into the mold from a tundish (not shown), and the molten steel is cooled within the mold to a half-solid steel which is drawn through a bottom opening of the mold. In this case, in order to maintain the level of the molten steel within the mold constant, it is necessary to detect the level of the molten steel.
• As illustrated in Fig. 1, a transmission coil 4 wound on a ferrite core is provided on the outside of the side wall 2 of the mold, and an AC voltage is applied to the coil 4 from an AC power supply 5 so as to produce alternating magnetic flux. A part of the alternating magnetic flux is distributed over and under the coil 4 and a part thereof enters into the side wall 2 of the mold made of a steel plate and transmits through the side wall 2 at its upper and lower portions. In addition, a very small part of the magnetic flux of which the density is small, penetrates the side wall 2 and passes through the molten steel 1 or air 6 or an intermediate portion including a boundary therebetween as shown in Fig. 1 depending on the level of the molten steel to reach the side wall 3 of the mold. Part of the magnetic flux which has reached the side wall 3 enters into the side wall 3 and passes through the side wall 3 at its upper and lower portions. Then, a very small part of the magnetic flux reaches the receiving coil 7 thereby inducing a voltage signal in the coil 7. The value of the induced signal is very small because a very small amount of the magnetic flux reaches the receiving coil 7. According to an experimental study by the inventors, the value of the induced voltage in the receiving coil 7 is about 1/1000 of the voltage value of the AC power supply 5. Therefore, the measurement in the field is easily affected by detrimental noise components which are irrelevant to the measuring signal of the molten steel level. Thus, the inventors have succeeded in removing the detrimental noise components, and practical use of this kind of detector. That is, the output of the receiving coil 7 is first passed through a band-pass filter 8 which allows only the same frequency as that of the AC power supply 5 to pass therethrough so that the detrimental noise components can be removed by the filter.
• The signal passed through the band-pass filter 8, however, may sometimes include a noise component of substantially the same frequency as that of the power supply. Thus, the phase of the AC power supply 5 is adjusted by a phase adjuster 11 to produce a signal having an adjusted phase matching that of the useful signal passed through the band-pass filter 8. The outputs of the band-pass filter 8 and the phase adjuster 11 are applied to a well-known synchronous detector 9 to extract from the output of the band-pass filter 8 only a component synchronized with the adjusted phase and to convert the component into a DC voltage signal. The components which are not synchronized with the adjusted phase (or undesired signals) are produced as AC voltages from the synchronous detector 9 and removed by a low-pass filter 10 which is capable of passing only a DC voltage and very low frequency components, so that only the useful signal of the DC signal component can be obtained. The phase adjuster 11 and the synchronous detector 9 may be any of well known types as disclosed, for example, in "Method of measurement of minute signal" in a catalogue of NF circuit design block, March, 1983.
• When the molten metal 1 is filled in the mold, part of the magnetic flux penetrated through the side wall 2 of the mold passes through the molten metal 1, thereby producing an eddy current within the molten metal. Since this eddy current is partially consumed in a form of Joule heat within the molten metal, the magnetic flux reaching the receiving coil 7 is very small.
• When no molten metal 1 is present within the mold, but only an air 6 is filled within it, no eddy current is produced in the molten steel unlike the above case. Thus, the magnetic flux is less attenuated within the mold, or is not lost in a form of joule heat, so that the amount of the magnetic flux reaching the receiving coil 7 is larger than that in the case in which the molten metal 1 is filled in the mold. Fig. 3 shows the relation between the measured value of the induced voltage in the receiving coil 7 and the level of the molten steel. In this graph, the width W of the mold is about 50 mm, the height (the dimension in the vertical direction) D of the rectangular cross-section of the transmission coil is about 40 mm, the mid point M in the graph indicates the induced voltage obtained when the level H2 of the molten steel coincides with the upper side of the transmission coil, and the maximum point A and the minimum point B in the graph indicate the induced voltages obtained when the molten metal is at a level H1 50 mm higher than H2 and at a level H3 50 mm lower than H2, respectively. In other words, the induced voltage is continuously changed as the level of the molten steel changes over the range of about 100 mm. Therefore, the change of the level of the molten steel over the range of about 100 mm can be detected by measuring this induced voltage. In the field of actual measurement, noise independent of the level of the molten steel is of course picked up, and hence the level of the molten steel is measured based on an output of a low-pass filter 10 shown in Fig. 1.
• When the level of the molten steel is greatly shifted from the position of the transmission or receiving coil, for example, separated by 50 mm or more, the position of the level cannot be detected by the above method even though it is possible to detect whether the level is above or below the transmission or receiving coil position. In that case, as shown in Fig. 2, a plurality of sets of transmission coils 4a, 4b, ..., 4n and the receiving coils 7a, 7b, ..., 7n are oppositely disposed on the outsides of the mold walls 2 and 3 within which the molten metal 1 is filled, at intervals in the vertical direction in which the level of the molten steel changes. The voltages of different frequencies from AC power supplies 5a, 5b, ..., 5n are supplied to the transmission coils 4a, 4b, ..., 4n, respectively. Parts of the alternating magnetic flux produced by the coils are transmitted through the molten steel 1 or air 6 and the mold walls 2, 3 so as to reach the receiving coils 5a, 5b, ..., 5n, thereby inducing AC voltages in the coils in the same manner as mentioned above. The induced voltages are respectively supplied through band-pass filters 8a, ..., 8n, and synchronous detectors 9a, ..., 9n to low pass filter 10a, 10b, ..., 10n in the same way as described above. In the circuit arrangement shown in Fig. 2, phase adjusters corresponding to the phase adjuster 11 shown in Fig. 1 are also provided for the respective coil sets, but for the sake of simplicity they are not shown.
• Moreover, the outputs of the low-pass filters 10a, 10b, ..., 10n are connected to binary encoders 12a, 12b, ..., 12n, respectively. Each of the binary encoders produces a logical value of "1" when the output voltage of the corresponding low-pass filter is lower than a value corresponding to point M in Fig. 1, and a logical value of "0" when it is equal to or higher than the value corresponding to point M. As a result, it is determined from the outputs of the binary encoders whether the level of the molten steel is higher or lower than the upper edge of the corresponding transmission coil, or the position H2 of the level of the molten steel in Fig. 3. By providing a plurality of sets of transmission and receiving coils at proper intervals to cover a range in which the level of the molten steel is possibly changed, which region of the range where the level of the molten steel is positioned is determined based on the outputs of the binary encoders.
• This method is able to determine which one of the regions, each corresponding to half the interval between adjacent two sets of transmission and receiving coils, the level of the molten steel is located, but unable to continuously detect the change of the level of the molten steel. If it is desired to continuously detect the change of the level of the molten steel over a wide range, a plurality of sets of transmission and receiving coils are disposed in such a manner that the interval between every two adjacent sets is equal to or slightly smaller than a distance between the points H1 and H3 in Fig. 3. Then, the position of the level of the molten steel can be precisely detected from the output voltages of the low-pass filters of adjacent two sets of transmission and receiving coils of which the outputs of the corresponding binary encodes are, respectively, "1" and "0". Fig. 4 is a graph showing the relation between the level of the molten steel and the output (induced voltage in the receiving coil) of each of the low-pass filter 10a, 10b, 10c, 10d of 4 sets of transmission and receiving coils arranged in this way.
• While in this embodiment the mold is made of a steel plate, it may be made of other metal or a material such as firebrick.
• Moreover, the molten metal to be measured may be other metal than steel, for example, aluminum.
• An embodiment of this invention will be described with reference to Figs. 5 to 7. In the arrangement shown in Fig. 5, the elements other than a phase difference detector 13, a cosine calculator 14 and a multiplier 15 are substantially the same as those in the apparatus of Fig. 1. The embodiment is an improvement in the detection sensitivity of the apparatus of Fig. 1. The phase of the AC power supply 5 is adjusted by a known phase adjuster 11 so that when no molten steel 1 is present within the mold, the phase of the useful AC signal, i.e. an output of the band-pass filter 8 is equal to that of the output of the phase adjuster 11, and hence the output of the phase difference detector 13 which converts the phase difference between them into a voltage signal is zero. When the output value of the phase difference detector 13 is zero, i.e. the phase difference $\text{Δφ = 0}$

  

  , $\text{Cos Δφ = 1}$

  

  is produced from a cosine calculator 14. Thus, the output of a known multiplier 15 which produces a product of the value of cos Δφ and the output of the low-pass filter 10 is equal to the output of the low-pass filter 10. On the other hand, when the molten steel 1 is filled within the mold, part of the magnetic flux having penetrated the side wall 2 of the mold causes an eddy current within the molten steel during passing through the molten steel 1, and it thus lost as joule heat. Consequently, the impedance of the receiving coil 7 equivalently represented by L (inductance) and R (resistance) is changed so that the phase of the voltage induced in the receiving coil is deviated by Δφ as compared with the case in which no molten metal 1 is present within the mold or only air is present. According to the experimental study by the inventors, when the frequency of the AC voltage of the AC power supply 5 is 600 Hz, the phase difference Δφ is 72 degrees. A signal corresponding to this phase difference Δφ is outputted from the phase difference detector 13 and converted into a value of Cos Δφ by the cosine calculator 14 as mentioned above. In case of Δφ being 72 degrees, Cos Δφ is 0.309 smaller than "1". As a result, the multiplier 15 produces a product of this value and the output value of the low-pass filter 10. Thus, the output value of the low-pass filter 10, which is smaller than a value of its output obtained when the molten metal is filled in the mold, is made further smaller by use of the multiplier 15. Generally, the value of the phase difference Δφ, or the output voltage of the phase difference detector 13 continuously increases as shown in Fig. 6, or decreases, as the level of the molten steel becomes higher in the vicinity of the position of the set of the transmission and receiving coils. In the above embodiment using the product of a value of Cos Δφ and the output value of the low-pass filter 10, the difference between the output of the low-pass filter 10 produced when the molten steel is filled in the mold and that produced when no molten steel exists is magnified by using the multiplier 15 as shown by solid line in Fig. 7 as compared with the case using no multiplier as shown by broken line, thus resulting in increasing the sensitivity.
• When the band width (frequency band) of the band-pass filter 8 is decreased, the noise signal detrimental to the measurement can be effectively decreased, but when it is too narrow, an error occurs in the measurement because a slight change of the frequency of the AC power supply 5 due to drift or the like may cause great shift in phase of the output of the band-pass filter when the frequency of the AC power supply 5 is changed from the center frequency of the band-pass filter 8. To avoid this effect, it is desired to make the band width wider or make the center frequency of the band-pass filter 8 slightly higher or lower than the frequency of the AC power supply 5.
• Moreover, when the time constant of the low-pass filter 10 is increased, more stability is achieved against the undesired noise signal, but the measurement time delay is increased. Thus, the desired time constant will be about 1/3 the allowable delay time (in this embodiment, the time constant is 30 msec).
• While in this embodiment the output of the phase difference detector 13 is zero when no molten steel 1 is present within the mold as described above, the same effect can be achieved by an arrangement in which the output of the phase difference detector 13 is made zero when the mold is filled with the molten steel 1, and a sine calculator is used in place of the cosine calculator 14.

Claims (14)

1. A method of detecting the level of molten metal

   (1) existing within a mold (2, 3), comprising the steps of:
      disposing the coils of at least one set of coils comprising
      a transmission coil (4) and a receiving coil (7) so as to oppose to each other with the mold

   (2, 3) interposed therebetween;
      applying an AC voltage (5) to said transmission coil of the said at least one set to produce alternating magnetic flux so that at least a part of said alternating magnetic flux passes through said mold and the molten metal, if any, and reaches said receiving coil of the said at least one set; and
      determining the level of said molten metal by sensing both the voltage value and the phase of the AC signal induced in said receiving coil (7) of the said at least one set by the part of the alternating magnetic flux.
2. A method according to claim 1, wherein the voltage value sensed for determining the level of said molten metal is the voltage value of the component of the AC signal in phase with the AC voltage (5) applied to said transmission coil (4) of the said at least one set.
3. A method according to claim 2, wherein the level of the molten metal is determined from the product of the voltage value of the said in-phase component of the AC voltage (5) and the sine or cosine of the phase difference between the phase of the AC signal induced in said receiving coil (7) of the said at least one set and the phase of the AC voltage (5) applied to said transmission coil (4) of the at least one set.
4. A method according to claim 1, wherein the level of said molten metal is determined by the phase difference between the phase of the AC signal induced in said receiving coil and the phase of the AC voltage (5) applied to said transmission coil (4) of the said at least one set.
5. A method according to any one of claims 1 to 4, wherein a plurality of sets of transmission coils (4a, 4b, ..., 4n) and receiving coils (7a, 7b, ..., 7n) are disposed at intervals in the vertical direction so as to cover the range over which the level of the molten metal can change.
6. A method according to any one of claims 1 to 5, wherein the said transmission coil (4) of the said at least one set is wound on a ferrite core.
7. A method according to any preceding claim, wherein the molten metal is steel within a mold of continuous casting equipment having at least two side walls.
8. Apparatus for detecting the level of molten metal

   (1) existing within a bold (2, 3) comprising:
      at least one set of coils comprising a transmission coil (4) and a receiving coil (7) disposed so as to oppose each other with said mold (2, 3) interposed therebetween;
      voltage applying means for applying an AC voltage (5) to said transmission coil (4) of the said at least one set to produce alternating magnetic flux so that at least a part of said alternating magnetic flux passes through the mold and the molten metal, if any, and reaches said receiving coil (7) of the said at least one set; and
      means (10, 13) for determining the level of the molten metal by sensing both the voltage value and the phase of an AC signal induced in said receiving coil (7) of the said at least one set by said alternating magnetic flux.
9. Apparatus according to claim 8, wherein the voltage value sensed in said level determining the voltage value of the component of the AC signal in phase with the AC voltage (5) applied to said transmission coil (4) of the said at least one set.
10. Apparatus according to claim 9, wherein the level of the molten metal is determined from the product of the voltage value of said in-phase component of the AC voltage and the sine or cosine of the phase difference between the phase of the AC signal induced in said receiving coil (7) of the said at least one set and the phase of the AC voltage (5) applied to said transmission coil (4) of the said at least one set.
11. Apparatus according to claim 8, wherein the level of said molten metal is determined from the voltage value of said AC signal and the phase difference between the phase of said AC signal and the phase of the AC voltage (5) applied to said transmission coil (4) of the at least one set.
12. Apparatus according to any one of claims 8 to 11, wherein a plurality of sets of transmission coils (4a, 4b, ..., 4n) and receiving coils (7a, 7b, ..., 7n) are disposed at intervals in the vertical direction so as to cover the range over which the level of the molten metal can change.
13. Apparatus according to any one of claims 8 to 12, wherein the said transmission coil (4) of the said at least one set is wound on a ferrite core.
14. Apparatus according to any one of claims 8 to 13, wherein the apparatus is arranged to determine the level of molten steel within a mold of continuous casting equipment having at least two side walls.

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